1. Field of the Invention
The present invention relates to a method of YC separation and a YC separation filter, especially to a 3-dimensional YC separation filter for use in a digital video separated in luminance and chrome signal from NTSC composite video signal by digital signal processing.
2. Description of the Related Art
FIG. 1 is a block diagram of YC separation filter according to the related art. An NTSC composite video signal is multiplexed with a base band luminance signal and a carrier color signal. A carrier signal modulates a sub carrier having frequency fsc by two types of color signal. Then the carrier signal is multiplexed in the high frequency range of the base band luminance signal. The composite video signal is, for example, separated into a luminance signal and a chromatic signal by YC separation filter to initiate independent signal processing on the luminance signal and the chromatic signal respectively.
In FIG. 1, a YC separation filter is composed of an AD converter 31, 2-dimensional YC separation filter 32, a reducer 33, frame memories 34 and 35, an amplifier 36, a switch 37, a motion detection circuit 38 and a reducer 39.
The AD converter 31 converts an input composite signal into digital signal (a composite video data) by sampling the composite signal with a clock signal synchronized with color burst signal having a frequency of 4 fsc. Then the composite video data is supplied to the 2-dimensional YC separation filter 32 which separates chromatic signal and provides the chromatic signal to one terminal of switch 37, and to the reducer 33, the motion detection circuit 38 and the frame memory 34 respectively.
The reducer 33 reduces an output of frame memory 34 from the composite video data, wherein the frame memory 34 delays the composite video data for one frame period. The chromatic signal is separated by the characteristic of comb filter in the frame period. The chromatic signal is amplified for half amplitude by the amplifier 36 and then supplied to the other terminal of switch 37. The output of frame memory 34 is supplied to the frame memory 35 and delayed for one frame period and supplied to the motion detection circuit 38.
The motion detection circuit 38 compares output signals from the AD converter 31, and the frame memories 34 and 35 and detects movement of an image signal. If the motion detection circuit 38 detects movement, the switch 37 selects the output from the 2-dimensional YC separation filter 32, which is a chromatic signal. If the motion detection circuit 38 detects no movement, the switch 37 selects the output from the amplifier 36, which is also a chromatic signal.
The switch 37 outputs chromatic signal and provides it to the reducer 39. The reducer 39 reduces the chromatic signal from the composite video data to separate and output a luminance signal.
A clock signal having frequency of 4 fsc synchronized with color burst signal for sampling a composite video signal meeting with NTSC standard is calculated as follows;4 fsc=4×(455/2)×fh=910×fh  (1)
(fh is a horizontal scanning frequency)
The delay timing for 525 horizontal scanning lines are necessary to obtain one frame delay. Accordingly, the amount of delay is calculated as follows;910[T/Line]×525[Line/Frame]=477750[T/Frame]  (2)
T indicated in the equation (2) represents sampling period 1/(4 fsc). The delay period of one frame can be obtained by 477750T delay.
As to digital video recorder such as DV format digital video camera and D-VHS (registered trademark) system, the clock signal of 13.5 MHz frequency is used and this clock frequency is independently generated from the input signal.
For example, it is supposed that this clock signal is used for YC separation filter, the filter can not obtain one frame delay of composite video data because the clock signal is not locked with the input composite signal.
The frequency of 13.5 MHz is 858 times of horizontal scanning frequency fh of NTSC format. Accordingly, the delay amount is indicated as;858×525=450450T′  (3)
(T′ is sampling period 1/13.5 MHz)
However, it is supposed that input composite signal of 1H (H is a horizontal scanning period) is indicated as 858.1T that the delay amount is indicated as;858.1×525=450502.5T′  (4)
In this sense, if the delay timing is fixed to 450450T′, the YC separation filter shown in FIG. 1 can not provide an appropriate operation of YC separation because the one frame delay timing is different from the actual delay timing for the input composite signal.